You have no champ for decrease def on all enemies, what is crucial vs. dungeon waves and in arena. So Stag Knight or Uugo would be my next choice. 

They need accuracy to land their debuffs and some speed to be faster than your damagedealers, so the latter allways attack with decrease def on the enemy. 

What areas are you looking specifically to improve in?

By the way, I wouldn't necessarily upgrade Mordecai to 60 yet as he can do his job well enough at 50.  The AOE HP will be just as effective at his current level, and the only thing you really gain is access to an accuracy banner, which you can probably overcome initially with a decent perception set.  He is an amazing champ and will take your spider to the next level, but he doesn't gain much from an upgrade.  

I use Tyrant Ixlimor for my HP burn on spider 18 and he is still level 50.

I don't see an unkillable team in your batch that would work with Demytha yet so I'd probably park her until you have the gear and champs.

Kael is great but you are probably past his use if you already have a campaign farmer (assumed to be Bellower?).  He could bring poisons to CB, Dragon and Ice Golem potentially.

That said, out of that list I'd probably throw my vote towards Stag Knight.  His AOE Decrease Def and Attack will help with doom tower as well as all dungeon waves leading up to the boss (plus the boss himself).  His A1 also brings 30% reduced speed.

Yeah, it is the accuracy thing I'm worried about... I think I need to be able to farm higher lvl spider to get some decent banners for accuracy.

Thanks for the advice!

I'm thinking I need to be able to farm spider better as my primary area. I think I'm too low a level farming to get decent banners for accuracy. I think that's the area I'm hurting for most. Knowing that Mordecai at lvl 50 for spider makes me lean more towards going for Stag Knight next.

I've had a lot of fun with Inithwe Bloodtwin on my main account.  His passive is ridiculous.

He doesn't need accuracy (unless you want the A3 leech for some reason), so he isn't too tricky to build.

Mordecai doesn't need any attack, crit rate or crit damage, so just focus on accuracy to ensure HP burn lands, and HP and defense to allow him to survive.  

You will also want to make sure he has decent speed, but don't go overboard.  Speed boots plus some addition speed here and there should be sufficient.  It is important for him to cycle back around to his HP burn, but enemies cannot have multiple HP burns on at the same time, so him being too fast won't change anything.